Rapid assessment of coral bleaching induced by 2023 ENSO in Karimunjawa, Central Java

Abstract The El Niño–Southern Oscillation (ENSO) event induces significant sea-level and temperature changes, which can severely impact the coral reef ecosystem. El Nino has affected Indonesia since June 2023, intensifying from late 2023 through April 2024, and has the potential to cause coral bleaching events. Karimunjawa is a vital marine protected area in the Java Sea, which has high coral diversity and is potentially threatened by mass bleaching. To assess the extent of coral bleaching in Karimunjawa waters, a rapid survey was conducted in early February 2024, focusing on the eastern waters, specifically the east coast of Karimunjawa Island and Tengah Island. The study of a 50 m line transect in different depths was implemented to quantify bleaching colony density and assess the bleaching level. The study revealed a low coral bleaching in Karimunjawa with varying severity, almost one-third of the coral colonies experienced >75% bleaching, and a quarter was found to have bleaching in both levels 50-75% and 25-50%. Coral bleaching densities ranged from 0.58 to 3.42 colonies per square meter, with the highest density in shallow waters, particularly for branching corals. Although coral bleaching in Karimunjawa is relatively low, the high percentage of Acropora corals experiencing bleaching will contribute to the decline in coral cover and diversity in this marine protected area.